Recently, various types of image display elements have been developed and, accordingly, there exist projection optical systems and projection-type image display apparatuses with a variety of applications and with a wide range of distances between the image display element and a screen that displays an image. For example, some projection optical systems and projection-type image display apparatuses using same are designed with a relatively small distance between the image display element and screen, including those where the projection-type image display apparatuses are preferably portable. The desire for portability of such projection-type image display apparatuses makes it desirable that they be small, which in turn requires a corresponding small size of the projection optical system.
In general, projection optical systems used in prior art projection-type image display apparatuses include combinations of lenses. However, the use of such combinations of lenses prevents the projection-type image display apparatuses from being as small as desired because of the optical path lengths required for enlargement and projection even when optical elements for folding the optical paths are provided in the projection optical systems.